The present invention provides a method to reduce the number of signal lines necessary for transmission of information on the maximum aperture and information on the minimum aperture of an objective to a camera body in a photographic camera of lens interchangeable type.
In a photographic camera provided with an automatic exposure control mechanism of shutter speed preset, i.e., priority, type, it is necessary to transmit information on the maximum aperture and information on the minimum aperture of the objective in use to the automatic exposure control mechanism. Assuming that a suitable type of film has been selected and a shutter speed has been set by the photographer, an appropriate exposure is sometimes impossible depending on the luminance of the object to be photographed, within an effective aperture range of the objective. In such a case, the photographer must be warned of this situation or photographing must be prevented, for example, by locking the shutter release. Especially when it is desired to provide a single-lens reflex camera usually used with a group of various interchangeable lenses with an automatic exposure control mechanism of shutter speed preset type, transmission of information on the maximum aperture and information on the minimum aperture is essential. With an automatic exposure control mechanism of diaphragm present, i.e., priority, type, on the other hand, it is necessary for a diaphragm value preset by the photographer to be transmitted to the automatic exposure control mechanism; transmission of the diaphragm value information is preferably done electrically in view of a fact that the essential part of the automatic exposure control mechanism comprises an electronic circuit and the camera itself is extremely small. Thus, a plurality of signal lines are required for transmission of information on the objective diaphragm to the camera body and, preferably, the number of these signal lines is as small as possible.